Economical ideas for lettered t-shirts

I make shirts, decals, signs, etc for a living. The best way to go is to buy a pack of inkjet shirts transfers (if you have an inkjet printer) If you have a laser printer look for the laser transfers. Staples, Target, Office Depot, etc. carries them. You will need an iron, and of course blank shirts. Blank shirts you can get in a multipack to save money. Make sure to follow the transfer instructions. If you need any help with the letters to print out, just let me know. Also if you go with these transfers you can add more details like a picture or logo. Again, if you need help I will help you out. Free of course.

Do you live near a Joann's Fabric Store? They have short sleeve T-shirts (including red ones) on sale 4 for $10, and 20% off fabric paints and markers. You could either paint Federer in freehand (the tubes of paint have applicator tops) or make stencils out of cardboard and then paint. Not sure what you mean by "different alphabets." Do you mean different fonts? In any case, same procedure. Good luck, sounds like a fun group.

I'd go for fusible web (sometimes also called fusible interfacing). Choose the type with paper backing.

Think of it as the fabric equivalent of double-sided tape:

You iron it to the back of the fabric you want to put on your t-shirts, cut out the shape, peel of the backing, position it where you want and iron again.

Now, a dedicated crafter would probably then run the whole thing through the sewing machine and add decorative stitching, but for a tennis match (assuming that's why you want FEDERER) or three, your iron-on letters will be more than sturdy enough. The webbing will prevent fraying and you can wash and probably even tumble-dry the t-shirts.

The main advantage is, that you don't have to worry about which paint or marker will be sufficiently opaque. A few yards of fusible web won't break the bank and you or another of your group probably have some old sheets or similar around, that you can cut up for the letters.

Buy plain T-shirts and a (black) marker felt pen.

Put cardboard between the front and back of the shirt when you put the letters on, so it will not bleed through.

On coloured shirts you can use white markers as well, but do test that the marker shows up before you spend a lot of money on it.